The standard three-way catalysts used in motor vehicles are not compatible with fuel-efficient lean-burn engines because they are unable to efficiently reduce the NOx pollutants created during combustion. Alumina-supported silver has been suggested as a catalyst for the reduction of NOx, but little is known about the catalytic mechanism.

Now Frédéric Thibault-Starzyk and colleagues in Caen with collaborators in the UK have developed1 a spectroscopic technique to monitor this reaction in situ. In the initial stages of the reaction NO and CO dissociate on the surface of the catalyst, producing a cyanide group bonded to a silver atom. A femtosecond laser pulse is used to heat the catalyst surface, triggering further reactions, and Fourier transform infrared spectroscopy is used to examine the adsorbed reaction intermediates.

Analysis of the infrared bands and changes in their intensity as the reaction proceeds reveal a key step in the mechanism. The cyanide group 'flips' from the silver to the alumina support. A band in the infrared spectra reveals that this occurs via a bridging intermediate — the cyanide bonds to an aluminium atom through its nitrogen before detaching from the silver. The results highlight the important role of the metal-support interface and offers opportunities for future catalyst development.